The invention relates to current limiting fuses of the type using cadmium fuse elements and usable as high voltage general purpose fuses.
The use of cadmium element fuses was proposed at least as early as 1915 as is evidenced by U.S. Pat. No. 1,157,919 of Oct. 26, 1915 which discloses a cadmium element cartridge fuse with a filler of silicic acid. A low voltage cartridge fuse for low current ratings having a single soldered cadmium wire element is disclosed in U.S. Pat. No. 3,529,270 of Sept. 15, 1970.
A cadmium element electrical fuse of the high voltage general purpose current limiting type is disclosed in U.S. Pat. No. 4,374,371 of Feb. 15, 1983, the disclosure of which is incorporated herein by reference. As disclosed, cadmium of purity between 95% and 99.999% is the material of choice for the fuse elements which are preferred to be of helical ribbon form, notched at spaced locations along the length of each ribbon and enclosed within a housing or cartridge substantially filled with spherical grains of high purity, random size quartz sand. A solution to the problem of sublimation of the high purity cadmium is addressed in the subsequent U.S. Pat. No. 4,413,246 of Nov. 1, 1983, the subject matter of which is also incorporated by reference herein. In this latter patent, it is taught to coat the elongate cadmium element with a metal having a melting temperature greater than that of cadmium and which does not significantly diffuse into the bulk of the cadmium element.
The use of cadmium elements as parallel-conducting electrical entities in high voltage general purpose current fuses has been found to eliminate most of the high cost features associated with current limiting fuses employing silver elements, cores, M spots, spark gaps and/or dual elements, in addition to offering superior performance.